Organic electroluminescence element

ABSTRACT

An organic electroluminescence element is disclosed which comprises a hole transporting layer containing a hole transporting material, a light emission layer containing a host compound and a phosphorescent compound, a hole blocking layer, and an electron transporting layer, the host compound having a band gap of from 3.3 eV to 5 eV, and having a molecular weight of not less than 500, and relationship c&lt;d being satisfied, wherein c (eV) represents a difference between energy level of LUMO (lowest unoccupied molecular orbital) in the hole blocking layer and energy level of LUMO in the light emission layer and d (eV) represents a difference between energy level of HOMO (highest occupied molecular orbital) in the hole blocking layer and energy level of HOMO in the light emission layer.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. patentapplication Ser. No. 10/410,312, filed on Apr. 9, 2003, the entirecontents of which are incorporated herein by reference. The Ser. No.10/410,312 application claimed the benefit of the date of the earlierfiled Japanese Patent Application No. JP 2002-110303 filed Apr. 12,2002.

FIELD OF THE INVENTION

This invention relates to an organic electro-luminescence (hereinafterreferred to also as organic EL) element, and particularly to an organicelectroluminescence element with excellent luminance of emitted lightand long lifetime.

BACKGROUND OF THE INVENTION

As an emission type electronic displaying device, there is anelectroluminescence device (ELD). As materials constituting the ELD,there is an inorganic electroluminescence element or an organicelectroluminescence element. The inorganic electroluminescence elementhas been used for a plane-shaped light source, but a high voltagealternating current has been required to drive the element. An organicelectroluminescence element has a structure in which a light emissionlayer containing a light emission compound is arranged between a cathodeand an anode, and an electron and a hole were injected into the lightemission layer and recombined to form an exciton. The element emitslight, utilizing light (fluorescent light or phosphorescent light)generated by deactivation of the exciton, and the element can emit lightby applying a relatively low voltage of from several to several decadevolts. Further, the element has a wide viewing angle and a highvisuality since the element is of self light emission type, and theelement is a complete solid element, and the element is noted from theviewpoint of space saving and portability.

However, in the organic EL element for practical use, an organic ELelement is required which efficiently emits light with high luminance ata lower power.

In U.S. Pat. No. 3,093,796, there is disclosed an element with longlifetime emitting light with high luminance in which stilbenederivatives, distyrylarylene derivatives or tristyrylarylene derivativesare doped with a slight amount of a fluorescent compound.

An element is known which comprises an organic light emission layercontaining an 8-hydroxyquinoline aluminum complex as a host compounddoped with a slight amount of a fluorescent compound (Japanese PatentO.P.I. Publication No. 63-264692), and an element is known whichcomprises an organic light emission layer containing an8-hydroxyquinoline aluminum complex as a host compound doped with aquinacridone type dye (Japanese Patent O.P.I. Publication No. 3-255190).

When light emitted through excited singlet state is used, the upperlimit of the external quantum efficiency (next) is considered to be atmost 5%, as the generation ratio of singlet excited species to tripletexcited species is 1:3, that is, the generation probability of excitedspecies capable of emitting light is 25%, and further, external lightemission efficiency is 20%. Since an organic EL element, employingphosphorescence through the excited triplet, was reported by PrinstonUniversity (M. A. Baldo et al., Nature, 403, 17, p. 151-154 (1998)),study on materials emitting phosphorescence at room temperature has beenactively made. As the upper limit of the internal quantum efficiency ofthe excited triplet is 100%, the light emission efficiency of the exitedtriplet is theoretically four times that of the excited singlet.Accordingly, light emission employing the excited triplet exhibits thesame performance as a cold cathode tube, and can be applied toillumination.

In order to improve luminance and emission lifetime of the organic ELelement, proposal has been made in which a hole blocking layer,inhibiting migration of holes from the light emission layer, is providedbetween the light emission layer and the cathode. This hole blockinglayer can efficiently accumulate holes in the light emission layer andimprove a recombination probability of electrons and holes, resulting inlight emission with high efficiency. It is reported, for example, inJapanese Patent O.P.I. Publication Nos. 8-109373 and 10-233284, that aphenanthroline derivative and a triazole derivative are effectively usedalone as a hole blocking material of the hole blocking layer. InJapanese Patent O.P.I. Publication No. 2001-28405 is disclosed anorganic El element with long lifetime in which a specific aluminumcomplex is used in the hole blocking layer. It has been reported (forexample, in Twelfth OyobutsuriGakkai Gakujutsukoen Kai Yokoshu 12-a-M7or in Pioneer Gijutsu Johoshi, Vol. 11, No. 1) that a red or green lightemission organic EL element employing a phosphorescent compound, when ahole blocking layer is incorporated in it, exhibits an inner quantumefficiency of approximately 100% and a lifetime of twenty thousandhours. However, there is room to be improved as for emission luminance.

There is an example in which a phosphorescent compound emitting a blueto blue-green color light is used as a dopant compound and a carbazolederivative such as CBP is used as a host compound, but the externalqauntum efficiency of this example is around 6%, which providesunsatisfactory results, although the phosphorescent compound is used(for example, Twelfth OyobutsuriGakkai Gakujutsukoen Kai Yokoshu12-a-M8, or Adachi et. al., “App. Phys. Lett., Vol. 79, p. 2082).

SUMMARY OF THE INVENTION

The present invention has been made in view of the aforementioned. Anobject of the present invention is to provide an organicelectroluminescence element with excellent emission luminance and longlifetime.

DETAILED DESCRIPTION OF THE INVENTION

The object of the invention has been attained by the followingconstitutions:

1-1. An organic electroluminescence element comprising a holetransporting layer containing a hole transporting material, a lightemission layer containing a host compound and a phosphorescent compound,a hole blocking layer, and an electron transporting layer, the hostcompound having a band gap of from 3.3 eV to 5.0 eV, and having amolecular weight of not less than 500, and relationship c<d beingsatisfied, wherein c (eV) represents a difference between energy levelof LUMO (lowest unoccupied molecular orbital) in the hole blocking layerand energy level of LUMO in the light emission layer and d (eV)represents a difference between energy level of HOMO (highest occupiedmolecular orbital) in the hole blocking layer and energy level of HOMOin the light emission layer.

1-2. The organic electroluminescence element of item 1-1 above, whereinthe hole transporting material has a ratio N/C of a nitrogen atom numberN to carbon atom number C of from 0.05 to 0.10.

1-3. The organic electroluminescence element of item 1-1 above, whereinrelationship 0.0 <c/d<0.4 is satisfied.

1-4. The organic electroluminescence element of item 1-1 above, in whichrelationship a >b is satisfied, wherein a (eV) represents a differencebetween energy level of LUMO in the light emission layer and energylevel of LUMO in the hole transporting layer and b (eV) represents adifference between energy level of HOMO in the light emission layer andenergy level of HOMO in the hole-transporting layer.

1-5. The organic electroluminescence element of item 1-4 above, whereinrelationship a≧b+0.2 is satisfied.

1-6. The organic electroluminescence element of item 1-1 above, whereinthe phosphorescent compound is an iridium complex, an osmium complex ora platinum complex.

1-7. The organic electroluminescence element of item 1-6 above, whereinthe phosphorescent compound is an iridium complex.

1-8. The organic electroluminescence element of item 1-6 above, whereinthe phosphorescent compound is an osmium complex or a platinum complex.

1-9. An organic electroluminescence element comprising a holetransporting layer containing a hole transporting material, a lightemission layer containing a host compound and a phosphorescent compound,a hole blocking layer, and an electron transporting layer, the holetransporting material having a ratio N/C of a nitrogen atom number N toa carbon atom number C of from 0.05 to 0.10, and relationship c<d beingsatisfied, wherein c (eV) represents a difference between energy levelof LUMO (lowest unoccupied molecular orbital) in the hole blocking layerand energy level of LUMO in the light emission layer and d (eV)represents a difference between energy level of HOMO (highest occupiedmolecular orbital) in the hole blocking layer and energy level of HOMOin the light emission layer.

1-10. The organic electroluminescence element of item 1-9 above, whereinrelationship 0.0<c/d<0.4 is satisfied.

1-11. The organic electroluminescence element of item 1-9 above, inwhich relationship a >b is satisfied, wherein a (eV) represents adifference between energy level of LUMO in the light emission layer andenergy level of LUMO in the hole transporting layer and b (eV)represents a difference between energy level of HOMO in the lightemission layer and energy level of HOMO in the hole transporting layer.

1-12. The organic electroluminescence element of item 1-11 above,wherein relationship a>b+0.2 is satisfied.

1-13. The organic electroluminescence element of item 1-9 above, whereinthe phosphorescent compound is an iridium complex, an osmium complex ora platinum complex.

1-14. The organic electroluminescence element of item 1-13 above,wherein the phosphorescent compound is an iridium complex.

1-15. The organic electroluminescence element of item 1-13 above,wherein the phosphorescent compound is an osmium complex or a platinumcomplex.

2-1. An organic electroluminescence element comprising a holetransporting layer, a light emission layer containing a host compoundand a phosphorescent compound, a hole blocking layer, and an electrontransporting layer, the host compound having a band gap of from 3.3 eVto 5.0 eV, and having a molecular weight of not less than 500, andrelationship c<d being satisfied, wherein c (eV) represents a differencebetween energy level of LUMO (lowest unoccupied molecular orbital) inthe hole blocking layer and energy level of LUMO in the light emissionlayer and d (eV) represents a difference between energy level of HOMO(highest occupied molecular orbital) in the hole blocking layer andenergy level of HOMO in the light emission layer.

2-2. An organic electroluminescence element comprising a holetransporting layer, a light emission layer containing a host compoundand a phosphorescent compound, a hole blocking layer, and an electrontransporting layer, a hole transporting material having a ratio N/C of anitrogen atom number N to carbon atom number C of from 0.05 to 0.10, andrelationship c<d being satisfied, wherein c (eV) represents a differencebetween energy level of LUMO (lowest unoccupied molecular orbital) inthe hole blocking layer and energy level of LUMO in the light emissionlayer and d (eV) represents a difference between energy level of HOMO(highest occupied molecular orbital) in the hole blocking layer andenergy level of HOMO in the light emission layer.

2-3. The organic electroluminescence element of item 2-1 or 2-2 above,wherein relationship 0.0<c/d<0.4 is further satisfied.

2-4. The organic electroluminescence element of any one of items 2-1through 2-3 above comprising a hole transporting layer, a light emissionlayer containing a host compound and a phosphorescent compound, a holeblocking layer, and an electron transporting layer, relationship a>bbeing satisfied, wherein a (eV) represents a difference between energylevel of LUMO in the light emission layer and energy level of LUMO inthe hole transporting layer and b (eV) represents a difference betweenenergy level of HOMO in the light emission layer and energy level ofHOMO in the hole transporting layer.

2-5. The organic electroluminescence element of item 2-4 above, whereinrelationship a>b+0.2 is further satisfied.

2-6. The organic electroluminescence element of any one of items 2-1through 2-5 above, wherein the phosphorescent compound is an iridiumcomplex.

2-7. The organic electroluminescence element of any one of items 2-1through 2-5 above, wherein the phosphorescent compound is an osmiumcomplex or a platinum complex.

The present invention will be detailed below.

In the invention, the phosphorescent compound is a compound in whichlight is emitted through light-excited triplet state in which twoelectron spins are in parallel with each other. Herein, thephosphorescent compound in the invention is considered to form exitedtriplet state at room temperature (from 15 to 30° C.) through energytransfer from the exited singlet state or excited triplet state of thefluorescent compound described above. Phosphorescent compounds have beenconsidered to be capable of emitting phosphoresce only at a lowtemperature such as 77K. However, since in recent years, compoundscapable of emitting phosphoresce at room temperature have been found,many compounds, for example,-heavy metal-containing complexes such asiridium complexes, have been mainly synthesized and studied (see forexample, S. Lamansky et al, J. Am. Chem. Soc., 123, pp. 4304, 2001).

An aluminum complex having 5 ligands, which has been recently noted, isused in a hole transporting layer, and markedly improves emissionlifetime of an organic EL element as compared with bathocuproine, buthas problem which lowers emission luminance. When in an organic ELelement comprising organic compounds with a wide band gap, the band gapof each of the compounds constituting the element is not optimized,unless a phosphorescent compound is used in a large amount as comparedwith a host compound, its effect is not sufficiently exhibited.

In view of the above, the present inventors have made an extensive studyon an organic EL element, and as a result, they have obtained desirableresult that optimization of the energy band structure of an organic ELelement forms a structure sufficiently accumulating holes and electronsat their recombination regions and provides high emission luminance andlong emission lifetime.

The present invention will be explained in detail below.

Band gap referred to in the invention is a difference between ionizationpotential and electron affinity of a compound. The ionization potentialand electron affinity are determined based on a vacuum level. Theionization potential is defined by energy necessary to release electronsof a compound existing in a HOMO (highest occupied molecular orbital)level to a vacuum level, while the electron affinity is defined byenergy released when electrons of a compound existing in a vacuum levelfall to a LUMO (lowest unoccupied molecular orbital) level and arestabilized.

In the invention, the band gap is obtained by vacuum depositing anorganic compound on a glass plate to obtain a deposit layer with athickness of 100 nm, measuring absorption spectra of the deposit layer,and determining wavelength Y (nm) at the longer absorption edge in theabsorption spectra in terms of X (eV), where the following formula isused.X=1239.8/Y

The ionization potential is directly measured employing a photoelectronspectroscopy, or can be also determined by correcting oxidationpotential electrochemically measured to a standard electrode.

In the invention, the ionization potential of an organic compound isdirectly measured employing a photoelectron spectroscopy, and is definedby a value obtained by being measured employing a low energy electronspectrometer Model AC-1 manufactured by Riken Keiki Co., Ltd.

The electron affinity is determined according to the followingdefinition-formula of the-band gap:(Band gap)=(ionization potential)−(electron affinity)

In the invention, energy level of HOMO is the same as ionizationpotential, and energy level of LUMO is the same as electron affinity.

The organic EL element of the invention is characterized in that itcomprises a hole transporting layer, a light emission layer containing ahost compound and a phosphorescent compound, a hole blocking layer, andan electron transporting layer, the host compound having a band gap offrom 3.3 eV to 5.0 eV, and having a molecular weight of not less than500, and relationship c<d being satisfied, wherein c (eV) represents adifference between energy level of LUMO in the hole blocking layer andenergy level of LUMO in the light emission layer and d (eV) represents adifference between energy level of HOMO in the hole blocking layer andenergy level of HOMO in the light emission layer. The molecular weightof the host compound is preferably not less than 600, and morepreferably from 600 to 2000.

Herein, “c” and “d” are represented by the following formulae:c=(energy level of LUMO in the hole blocking layer)−(energy level ofLUMO in the light emission layer)d=(energy level of HOMO in the hole blocking layer)−(energy level ofHOMO in the light emission layer)

In the organic EL element having the energy band structure describedabove, the hole blocking function of the element is sufficientlyperformed, which contributes to improvement of emission efficiency.Particularly, the wide band gap of the host compound contributes toimprovement of emission efficiency of blue light which is a light with ashorter wavelength. Further, the host compound with a molecular weightof not less than 500, and preferably not less than 600 increases heatresistance of the organic EL element and markedly increases lifetime ofthe organic EL element.

In the invention, the organic EL element is characterized in that itcomprises a hole transporting layer containing a hole transportingmaterial, a light emission layer containing a host compound and aphosphorescent compound, a hole blocking layer, and an electrontransporting layer, a hole transporting material having a ratio N/C of anitrogen atom number N to carbon atom number C of from 0.05 to 0.10, andrelationship c<d being satisfied, wherein c (eV) represents a differencebetween energy level of LUMO in the hole blocking layer and energy levelof LUMO in the light emission layer and d (eV) represents a differencebetween energy level of HOMO in the hole blocking layer and energy levelof HOMO in the light emission layer.

In the organic EL element having the energy band structure describedabove, the hole blocking function of the element is sufficientlyperformed, which contributes to improvement of emission efficiency.However, even if the energy band structure is proper, when holetransporting function of a hole transporting material in the holetransporting layer is poor, it cannot contribute to improvement ofemission efficiency. The present inventors have made an extensive study,and as a result, they have found that improvement of emission efficiencyis realized by limiting the N/C ratio of the hole transporting materialto a specific range, and completed the invention.

In the organic EL element of the invention, it is preferred thatrelationship 0.0<c/d≦0.4 is further satisfied. Hole blocking function ofthe organic EL element is further enhanced by limiting the c/d to therange as described above.

The organic EL element of the invention comprises a hole transportinglayer, a light emission layer containing a host compound and aphosphorescent compound, a hole blocking layer and an electrontransporting layer, and is characterized in that the relationship a>b issatisfied, wherein a (eV) represents a difference between energy levelof LUMO in the light emission layer and energy level of LUMO in the holetransporting layer and b (eV) represents a difference between energylevel of HOMO in the light emission layer and energy level of HOMO inthe hole transporting layer.

Herein, “a” and “b” are represented by the following formulae:a=(energy level of LUMO in the light emission layer)−(energy level ofLUMO in the hole transporting layer)b=(energy level of HOMO in the light emission layer)−(energy level ofHOMO in the hole transporting layer)

It is preferred that relationship a≧b+0.2 is satisfied. In the organicEL element having such an energy band structure, the hole transportinglayer of the element can sufficiently perform an electron blockingfunction.

In the invention, the light emission layer comprises a host compound anda phosphorescent compound (light emission material). Examples of thehost compound include an aligoarylene derivative, a boron-containingcompound, a styryl type compound, a triarylamine derivative, quinazolineor its derivatives, carbazole or its derivatives and triazine or itsderivatives. Of these, a boron-containing compound, and carbazole or itsderivatives are preferred. The phosphorescent compound in the inventionis preferably a metal complex compound containing, as a center metal, ametal belonging to a group VIII of the periodic table, more preferably ametal complex compound containing osmium, iridium or platinum, and mostpreferably an iridium complex.

Examples of the phosphorescent compound in the invention will be listedbelow, but are not limited thereto.

The doping amount of the phosphorescent compound in the invention isfrom more than 0% to less than 30% by weight, preferably from 0.1% to20% by weight, and more preferably from 6% to less than 15% by weight,based on the host compound.

When a material in each layer of an organic EL element is employed sothat the energy band structure defined in the invention is formed, theeffect described in the invention is sufficiently exhibited, even if thephosphorescent compound is doped at the low doping amount as less than6% by weight.

Constitution of the organic electroluminescence element of the inventionwill be detailed below.

The organic EL element in the invention comprises a hole transportinglayer, a light emission layer a hole blocking layer, an electrontransporting layer, and optionally an anode buffer layer and a cathodebuffer layer, which are provided between a cathode and an anode.

In the invention, the preferred structure of the organic EL element isshown below, but the invention is not limited thereto.

-   (i) Anode/Hole transporting layer/Light emission layer/Hole blocking    layer/Electron transporting layer/Cathode-   (ii) Anode/Anode buffer layer/Hole transporting layer/Light emission    layer/Hole blocking layer/Electron transporting layer/Cathode buffer    layer/Cathode

The above light emission layer is a layer where electrons and holes,injected from electrodes, an electron transporting layer or a holetransporting layer, are recombined to emit light. The portions wherelight emits may be portions in the light emission layer or portions atthe interface between the light emission layer and the layer adjacentthereto. The light emission materials (for example, phosphorescentcompounds) of the light emission layer may have a hole transportingcapability or an electron transporting capability as well as a lightemission capability. Most of hole transporting materials used in thehole transporting layer, hole blocking materials used in the holeblocking layer, and electron transporting materials used in the electrontransporting layer, can be used as light emission materials.

The light emission layer can be formed employing a known method such asa vacuum deposition method, a spin coat method, a casting method and aLangumiur-Blodgett method (LB method). The thickness of the lightemission layer is not specifically limited, but is ordinarily from 5 nmto 5 μm. The light emission layer may be composed of a single layercomprising one or two or more of light emission materials, or of plurallayers comprising the same composition or different composition.

The light emission layer can be formed by the method such as thatdescribed in Japanese Patent O.P.I. Publication No. 57-51781, in which alight emission material is dissolved in a solvent together with a bindersuch as a resin, and the thus obtained solution is formed into a thinlayer by a method such as a spin-coat method. Thickness of the emissionlayer thus formed is not specially restricted. Although the thickness ofthe layer thus formed is optionally selected, the thickness ispreferably within the range of from 5 nm to 5 μm.

The hole transporting layer, hole blocking layer, and electrontransporting layer will be explained below.

The hole transporting layer has a function of transporting the holeinjected from the anode to the light emission layer. Many holes can beinjected in a lower electric field by the presence of the holetransporting layer between the anode and the light emission layer.Moreover, an element can be obtained which increases a light emissionefficiency and has an excellent light emission ability, since theelectrons injected into the light emission layer from the cathode bufferlayer or the electron transporting layer are accumulated at theinterface in the light emission layer by a barrier to electrons existingat the interface between the light emission layer and the holetransporting layer. The hole transporting material used in the holetransporting layer can be optionally selected from known materialswithout any limitation as far as they have the property defined in theinvention. Such a material include those employed for hole injecting ortransporting materials in conventional photoconductive elements or knownmaterials used in the hole transporting layer of conventional organic ELelements.

The hole transporting material described above may be either an organicsubstance or an inorganic substance as long as it has a holetransporting ability or an ability to form a barrier to electron.Examples of the hole transporting material include a triazolederivative, an oxadiazole derivative, an imidazole derivative, apolyarylalkane derivative, a pyrazoline derivative and a pyrazolonederivative, a phenylenediamine derivative, an arylamine derivative, anamino substituted chalcone derivative, an oxazole derivative, a styrylanthracene derivative, a fluorenone derivative, a hydrazone derivative,a stilbene derivative, a silazane derivative, an aniline copolymer, andan electroconductive oligomer, particularly a thiophene oligomer. As thehole transporting material, those described above are used, but aporphyrin compound, an aromatic tertiary amine compound, or astyrylamine compound is preferably used, and an aromatic tertiary aminecompound is more preferably used. A polymer can be used which containsthe above materials in the side chain or in the main chain.

As the hole transporting material, inorganic compounds such as p-Si andp-SiC are usable. The hole transporting layer can be formed by layeringthe hole transporting material by a known method such as a vacuumdeposition method, a spin coat method, a casting method and an LBmethod. The thickness of the hole transporting layer is not specificallylimited, but is ordinarily from 5 nm to 5 μm. The hole transportinglayer may be composed of a single layer comprising one or two or more ofthe materials mentioned above, or of plural layers comprising the samecomposition or different composition.

The hole blocking layer has a function inhibiting migration of holesinjected from the anode. Probability of recombination of holes andelectrons is increased by the presence of the hole transporting layerbetween the electron transporting layer and the light emission layer,which provides an element with an excellent light emission abilityincreasing a light emission efficiency. Examples of hole blockingmaterials used in the hole blocking layer include phenanthroline or itsderivatives, triazole or its derivatives, aluminum complex (BAlq) with 5ligands, an aligoarylene derivative, a styryl type compound, quinazolineor its derivatives, oxadiazole or its derivatives, pyrimidine or itsderivatives, triazine or its derivatives, and a boron-containingcompound. Of these, phenanthroline or its derivatives, an aligoarylenederivative, a styryl type compound, quinazoline or its derivatives, anda boron-containing compound are preferred.

The electron transporting layer has a function of transporting electronsinjected to the cathode to the light emission layer. Many electrons areinjected into the light emission layer at a lower electric field by thepresence of the electron transporting layer between the cathode and thelight emission layer. Examples of the electron transporting materialused in the electron transporting layer include a nitro-substitutedfluorene derivative, a diphenylquinone derivative, a thiopyran dioxidederivative, a heterocyclic tetracarboxylic acid anhydride such asnaphthaleneperylene, a carbodiimide, a fluolenylidenemethane derivative,an anthraquinodimethane an anthrone derivative, an oxadiazolederivative, a triazole derivative and a phenanthroline derivative.Moreover, a thiadiazole derivative which is formed by substituting theoxygen atom in the oxadiazole ring of the foregoing oxadiazolederivative with a sulfur atom, and a quinoxaline derivative having aquinoxaline ring known as an electron withdrawing group are usable asthe electron transporting material. A polymer can be used which containsthe above materials in the side chain or in the main chain.

A metal complex of an 8-quinolynol derivative such as aluminumtris-(8-quinolynol) (Alq₃), aluminum tris-(5,7-dichloro-8-quinolynol),aluminum tris-(5,7-dibromo-8-quinolynol), aluminumtris-(2-methyl-8-quinolynol), aluminum tris-(5-methyl-8-quinolynol), orzinc bis-(8-quinolynol) (znq₂), and a metal complex formed by replacingthe central metal of the foregoing complexes with another metal atomsuch as In, Mg, Cu, Ca, Sn, Ga or Pb, can be used as the electrontransporting material. Furthermore, a metal free or metal-containingphthalocyanine, and a derivative thereof, in which the molecularterminal is replaced by a substituent such as an alkyl group or asulfonic acid group, are also preferably used as the electrontransporting material. The distyrylpyrazine derivative exemplified as amaterial for the light emission layer may preferably be employed as theelectron transporting material. An inorganic semiconductor such as n-Siand n-SiC may also be used as the electron transporting material in asimilar way as in the hole transporting layer.

An electron transporting layer can be formed by layering the compoundsdescribed above by a known method such as a vacuum deposition method, aspin coat method, a casting method and an LB method. The thickness ofthe electron transporting layer is not specifically limited, but isordinarily from 5 nm to 5 μm. The electron transporting layer may becomposed of a single layer comprising one or two or more of thematerials mentioned above, or of plural layers comprising the samecomposition or different composition.

A buffer layer (an electrode interface layer) may be provided betweenthe anode and the hole transporting layer, or between the cathode andthe electron transporting layer.

The buffer layer is a layer provided between the electrode and anorganic layer in order to reduce the driving voltage or to improve lightemission efficiency. As the buffer layer there are an anode buffer layerand a cathode buffer layer, which are described in “Electrode Material.page 123, Div. 2 Chapter 2 of “Organic EL element and its frontier ofindustrialization” (published by NTS Corporation, Nov. 30, 1998) indetail.

The anode buffer layer is described in Japanese Patent O.P.I.Publication Nos. 9-45479, 9-260062, and 8-288069 etc., and its examplesinclude a phthalocyanine buffer layer represented by a copperphthalocyanine layer, an oxide buffer layer represented by a vanadiumoxide layer, an amorphous carbon buffer layer, a polymer buffer layeremploying an electroconductive polymer such as polyaniline (emeraldine),and polythiophene, etc.

The cathode buffer layer is described in Japanese Patent O.P.I.Publication Nos. 6-325871, 9-17574, and 9-74586, etc. in detail, and itsexamples include a metal buffer layer represented by a strontium oraluminum layer, an alkali metal compound buffer layer represented by alithium fluoride layer, an alkali earth metal compound buffer layerrepresented by a magnesium fluoride layer, and an oxide buffer layerrepresented by an aluminum oxide or lithium oxide layer.

In the organic EL element of the invention, presence of the cathodebuffer layer is preferable in reducing the driving voltage or improvinglight emission efficiency.

The buffer layer is preferably very thin and has a thickness ofpreferably from 0.1 to 100 nm depending on kinds of the material used.

Electrodes of the organic EL element will be explained below. Theelectrodes consist of a cathode and an anode.

For the anode of the organic EL element, a metal, an alloy, or anelectroconductive compound each having a high working function (not lessthan 4 eV), and mixture thereof are preferably used as the electrodematerial. Concrete examples of such an electrode material include ametal such as Au, and a transparent electroconductive material such asCuI, indium tin oxide (ITO), SnO₂, or ZnO.

The anode may be prepared by forming a thin layer of the electrodematerial according to a depositing or sputtering method, and by formingthe layer into a desired pattern according to a photolithographicmethod. When required precision of the pattern is not so high (not lessthan 100 μm), the pattern may be formed by evaporating or sputtering ofthe electrode material through a mask having a desired form. When lightis emitted through the anode, the transmittance of the anode ispreferably 10% or more, and the sheet resistivity of the anode ispreferably not more than several hundred n/Fl. The thickness of theanode is ordinarily within the range of from 10 nm to 1 μm, andpreferably from 10 to 200 nm, although it may vary due to kinds ofmaterials used.

On the other hand, for the cathode, a metal (also referred to as anelectron injecting metal), an alloy, and an electroconductive compoundeach having a low working function (not more than 4 eV), and a mixturethereof is used as the electrode material. Concrete examples of such anelectrode material include sodium, sodium-potassium alloy, magnesium,lithium, a magnesium/copper mixture, a magnesium/silver mixture, amagnesium/aluminum mixture, magnesium/indium mixture, analuminum/aluminum oxide (Al₂O₃) mixture, indium, a lithium/aluminummixture, and a rare-earth metal. Among them, a mixture of an electroninjecting metal and a metal higher in the working function than that ofthe electron injecting metal, such as the magnesium/silver mixture,magnesium/aluminum mixture, magnesium/indium mixture, aluminum/aluminumoxide (Al₂O₃) mixture or lithium/aluminum mixture, is suitable from theview point of the electron injecting ability and resistance tooxidation.

For materials for the cathode used in the organic EL element of theinvention, an aluminum alloy is preferably used, and the aluminum alloycontains aluminum in an amount of preferably from 90% to less than 100%by weight, and more preferably from 95% to less than 100% by weight.This can provide an organic EL element with high luminance and longemission lifetime.

The cathode can be prepared forming a thin layer of such an electrodematerial by a method such as a deposition or sputtering method. Thesheet resistivity of the cathode is preferably not more than severalhundred Ω/□, and the thickness of the cathode is ordinarily from 10 nmto 1 μm, and preferably from 50 to 2,00 nm. It is preferable inincreasing the light emission efficiency that either the anode or thecathode of the organic EL element is transparent or translucent.

A substrate preferably employed for the organic electroluminescenceelement of the invention is not restricted to specific kinds ofmaterials such as glass and plastic, as far as it is transparent.Examples of the substrate preferably employed used in the organicelectroluminescence element of the invention include glass, quartz andlight transmissible plastic film.

Examples of the light transmissible plastic film include films such aspolyethyleneterephthalate (PET), polyethylenenaphthalate (PEN),polyethersulfone (PES), polyetherimide, polyetheretherketone,polyphenylenesulfide, polyarylate, polycarbonate (PC), cellulosetriacetate (TAC), cellulose acetate propionate (CAP) and so on.

Preferable examples in the preparation of the organic EL element will bedescribed below.

For one example, preparation of the EL element having the foregoingconstitution, Anode/Hole transporting layer/Light emission layer/Holeblocking layer/Electron transporting layer/Cathode buffer layer/Cathodewill be described.

A thin layer of a desired material for electrode such as a material ofthe anode is formed on a suitable substrate by a deposition orsputtering method, so that the thickness of the layer is not more than 1μm, and preferably within the range of from 10 to 200 nm to prepare theanode. Then, the hole transporting layer, the light emission layer, thehole blocking layer, the electron transporting layer and the cathodebuffer layer are formed on the resulting anode in that order.

For methods for forming the layers described above, there are a spincoating method, a casting method and a deposition method. A vacuumdeposition method or a spin coat method is preferable since a uniformlayer can be formed and a pinhole is formed with difficulty. Differentlayer forming methods may be used on forming different layers. Althoughconditions of the vacuum deposition are different due to kinds ofmaterials used, or an intended crystalline or association structure ofthe molecular deposited layer, the vacuum deposition is preferablycarried out at a boat temperature of from 50° C. to 450° C., at a vacuumdegree of from 10⁻⁶ to 10⁻³ Pa, at a deposition speed of from 0.01 to 50nm/second, and at a substrate temperature of from −50 to 300° C., toform a layer thickness of from 5 nm to 5 μm.

After formation of these layers, a layer comprising a material forcathode is formed thereon by, for example, a deposition method orsputtering method so that the thickness is not more than 1 μm, andpreferably from 50 to 200 nm, to provide the cathode. Thus a desiredorganic EL element is obtained. It is preferred that the layers from thehole injecting layer to the cathode are continuously formed under onetime of vacuuming to prepare the organic EL element. However, on the wayof the layer formation under vacuum a different layer formation methodmay be inserted. When the different layer formation method is used, themethod is required to be carried out under a dry inert gas atmosphere.

Further, the organic EL element can be prepared in the reverse order, inwhich the cathode, the cathode buffer layer, electron transportinglayer, the hole blocking layer, the light emission layer, the holetransporting layer, and the anode are formed in that order. Lightemission can be observed when a direct current with a voltage of fromabout 5 to 40 V is applied to the thus prepared organic EL element sothat the polarity of the anode is positive and that of the cathode isnegative. When the voltage is applied in the reverse polarity, nocurrent is formed and light is not emitted at all. When an alternatingcurrent is applied, light is emitted only when the polarity of the nodeis positive and that of the cathode is negative. The shape of the waveof the alternating current may be optionally selected.

The organic EL element of the invention may be used as a lamp such as aroom light or a light source for exposure, a projection deviceprojecting an image or a display directly viewing a still image or amoving image. When the element is used as a display for reproducing amoving image, the driving method may be either a simple matrix (passivematrix) method or an active matrix method. When two or more kinds of theorganic EL element of the invention are used which have differentemission light color, a full color display can be prepared.

EXAMPLES

The present invention will be explained in the following examples, butis not limited thereto.

Example 1

<Preparation of Organic EL Element>

(Preparation of Organic EL Element Sample OLED 1)

A pattern was formed on a substrate (manufactured by NH Technoglass Co.,Ltd.) composed of a glass plate and a 150 nm ITO (indium tin oxide)layer as an anode. Then the resulting transparent substrate having theITO transparent electrode was subjected to ultrasonic washing ini-propyl alcohol and dried by a dry nitrogen gas and subjected toUV-ozone cleaning for 5 minutes. Thus obtained transparent substrate wasfixed on a substrate holder of a vacuum deposition apparatus availablein the market.

After the pressure in the vacuum tank was reduced to 4×10⁻⁴ Pa, copperphthalocyanine was deposited on the ITO layer to give a hole injectinglayer with a thickness of 10 nm, and then α-NPD was deposited to give ahole transporting layer with a thickness of 30 nm. A heating boatcarrying CBP and a heating boat carrying exemplified compound Ir-10 wereindependently heated by supplying an electric current to both boats todeposit CBP and Ir-10 onto the hole transporting layer at a depositingspeed ratio of CBP to Ir-10 of 100:6 to form a light emission layer witha thickness of 30 nm.

A heating boat carrying BC was heated by supplying an electric currentto form a hole blocking layer with a thickness of 10 nm, and a heatingboat carrying Alq₃ was heated by supplying an electric current to formthereon an electron transporting with a thickness of 20 nm. Further, 1nm lithium fluoride layer was deposited on the electron transportinglayer and a 100 nm aluminum layer was provided on the lithium fluoridelayer. Thus, organic EL element sample OLED 1 (comparative) wasprepared. This element sample emitted blue light resulting from Ir-10.

(Preparation of Organic EL Element Samples OLED 2 Through 18)

Organic EL element samples OLED 2 through 18 were prepared in the samemanner as comparative organic EL element sample OLED 1, except thatcompound α-NPD in the hole transporting layer, host compound CBP in thelight emission layer or compound BC in the hole blocking layer wasreplaced with those as shown in Table 1.

(Measurement of Characteristic Value of Each Compound)

<Measurement of Band Gap of the Host Compound>

Each host compound used in the above organic EL element samples OLED 1through 18 is deposited on a glass substrate to form a 100 nm depositlayer, and absorption spectra of the resulting deposit layer aremeasured. Band gap X (eV) is determined a band gap X (eV) from awavelength Y (nm) at the longer absorption edge in the absorptionspectra employing the following formula:X=1239.8/Y.

The results are shown in Table 1

<Energy Level (eV) of LUMO of Each Compound and Energy Level (eV) ofHOMO (Highest Occupied Molecular Orbital of Each Compound>

Energy level (eV) of LUMO and energy level (eV) of HOMO (highestoccupied molecular orbital of each compound used in the above organic ELelement samples OLED 1 through 18 are shown in Table 2. TABLE 2 Energylevel (eV) Compound HOMO LUMO α-NPD 5.4 2.3 HMTPD 5.4 2.1 m-MTDATXA 5.42.2 TPD1 5.6 2.3 TPD2 5.7 2.1 CBP 6.0 2.9 BC 6.4 2.9 TAZ 6.4 2.4 TCTA5.9 2.7 Compound 1 6.4 2.6 Compound 2 6.0 2.6 Compound 3 6.1 2.5Compound 4 5.7 2.5 Compound 5 6.5 2.6 Compound 6 6.5 2.5 Alq₃ 5.8 3.1

In Table 1, N/C represents a nitrogen atom number in the holetransporting material/a carbon atom number in the hole transportingmaterial, “a” represents (a LUMO energy level (eV) in the light emissionlayer—a LUMO energy level (eV) in the hole transporting layer), “b”represents (a HOMO energy level (eV) in the light emission layer—a HOMOenergy level (eV) in the hole transporting layer), “c” represents (aLUMO energy level (eV) in the hole blocking layer—a LUMO energy level(eV) in the light emission layer), and “d” represents (a HOMO energylevel (eV) in the hole blocking layer—a HOMO energy level (eV) in thelight emission layer).

The chemical structures of the compounds used above are shown below.

<<Evaluation of Characteristics of Organic EL Element Samples>>

When a direct current voltage of 10V was applied to each of theresulting organic EL element samples at 23° C. in an atmosphere of adried nitrogen gas, luminance (cd/m²) of light emitted from the sampleand time (hereinafter referred to also as luminance half-life) takenuntil the luminance was reduced to half were measured. The luminance oflight emitted from the organic EL element sample Nos. 2 through 18 wasexpressed by a relative value (hereinafter referred to also as relativeluminance) when the luminance of light emitted from the organic ELelement sample No. 1 was set at 100. The luminance half-life of lightemitted from the organic EL element sample Nos. 2 through 18 wasexpressed by a relative value (hereinafter referred to also as relativeluminance half-life) when the luminance half-life of light emitted fromthe organic EL element sample No. 1 was set at 100. The luminance(cd/m²) was measured according to CS-1000 produced Minolta Co., Ltd. Theresults are shown in Table 3. TABLE 3 Evaluation results Sample Relativeluminance No. Relative luminance half-life Remarks 1 100 100 Comparative2 105 80 Comparative 3 105 83 Comparative 4 110 107 Comparative 5 112100 Comparative 6 110 95 Comparative 7 148 172 Invention 8 145 153Invention 9 149 190 Invention 10 155 170 Invention 11 149 173 Invention12 141 148 Invention 13 144 145 Invention 14 158 195 Invention 15 160194 Invention 16 150 162 Invention 17 163 170 Invention 18 165 173Invention

As is apparent from Table 1 and 3 above, inventive organicelectroluminescence element samples comprising the hole transportingmaterial in the invention, the light emission material in the invention,and the hole blocking material in the invention exhibited an excellentresult in luminance and emission lifetime, compared with comparativesamples.

Example 2

Organic EL element samples OLED 7G through OLED 18G were prepared in thesame manner as in organic EL element samples OLED 7 through OLED 18 ofExample 1, respectively, except that Ir-1 was used instead of Ir-10.Further, organic EL element samples OLED 7R through OLED 18R wereprepared in the same manner as in organic EL element samples OLED 7through OLED 18 of Example 1, respectively, except that Ir-9 was usedinstead of Ir-10. The resulting samples were evaluated for luminance andluminance half-life in the same manner as in Example 1. As a result,inventive organic electroluminescence element samples exhibited anexcellent result in luminance and emission lifetime. Green light wasemitted from the samples employing Ir-1, and red light was emitted fromthe samples employing Ir-9.

EFFECT OF THE INVENTION

The present invention can provide an organic electroluminescence elementwith excellent luminance and with long lifetime.

1. An organic electroluminescence element comprising a hole transportinglayer containing a hole transporting material, a light emission layercontaining a host compound and a phosphorescent compound, a holeblocking layer, and an electron transporting layer, the holetransporting material having a ratio N/C of a nitrogen atom number N toa carbon atom number C of from 0.05 to 0.10, and relationship c<d beingsatisfied, wherein c (eV) represents a difference between energy levelof LUMO (lowest unoccupied molecular orbital) in the hole blocking layerand energy level of LUMO in the light emission layer and d (eV)represents a difference between energy level of HOMO (highest occupiedmolecular orbital) in the hole blocking layer and energy level of HOMOin the light emission layer.
 2. The organic electroluminescence elementof claim 1, wherein relationship 0.0<c/d≦0.4 is satisfied.
 3. Theorganic electroluminescence element of claim 1, in which relationshipa>b is satisfied, wherein a (eV) represents a difference between energylevel of LUMO in the light emission layer and energy level of LUMO inthe hole transporting layer and b (eV) represents a difference betweenenergy level of HOMO in the light emission layer and energy level ofHOMO in the hole transporting layer.
 4. The organic electroluminescenceelement of claim 3, wherein relationship a≧b+0.2 is satisfied.
 5. Theorganic eletroluminescence element of claim 1, wherein thephosphorescent compound is an iridium complex, an osmium complex or aplatinum complex.
 6. The organic eletroluminescence element of claim 5,wherein the phosphorescent compound is an iridium complex.
 7. Theorganic electroluminescence element of claim 5, wherein thephosphorescent compound is an osmium complex or a platinum complex.